Shock absorbers mechanically linked to springs of automobiles, trucks and other vehicles are used to dampen the free vibrations of the axles and wheels of the vehicles. These free vibrations, if not restricted by the use of shock abosrbers cause several undesirable conditions. The comfort of the vehicle rider suffers and mechanical damage to the vehicle can result under certain road and vehicle conditions. For example, when a vehicle is traveling on a road having regularly spaced bumps, the vehicle at low speeds will experience an extreme pitching motion and at high speeds will experience extreme vibration of the wheels and axles. These conditions result from the interaction of the road-forced vibrations with the vehicle mass-spring system, and the wheel and axle mass-spring system reaching resonance at different vibrations and speed ratios.
The shock absorbers typically used in vehicles are hydraulic and operate on the dashpot principle, a piston moving in a cylinder filled with oil. Any relative motion between the axle and the vehicle body results in the operation of the shock absorber. This operation includes oil leaking past the piston and cylinder or passing through a spring controlled valve which opens only when a certain predetermined pressure difference exists on the two sides of the piston. Most shock absorbers utilize one-way valves thereby creating different dampening results depending upon whether there is compression or expansion of the shock absorber. This difference in dampening is accomplished by forcing the oil through different sets of apertures by means of check valves. Usually the configuration is such that when the vehicle body and axle are spreading apart, the dampening is greater than when the vehicle and axle are coming together.
Generally, in shock absorbers of single tube design, all valving is contained in the piston of the shock absorber. One-way leaf spring valves control oil flowing through orifices from one side of the piston to the other. On compression, oil is forced by the valving to flow from the high pressure side of the piston to the low pressure side through a certain set of orifices. On extension, the valving forces the oil to flow in the opposite direction through another set of orifices. Dampening in each direction, compression or extension is then controlled by the size and number of orifices available for flow in each direction. Single cylinder shock absorbers suffer from the need for a vapor space in the cylinder to allow for piston rod displacements. This lack of vapor space causes erratic action as air mixes with the oil passing through the valve.
In two cylinder shock absorbers, means are provided to help eliminate the problems caused by the lack of a vapor space in the single cylinder shock absorbers. The inner cylinder acts as a single cylinder shock absorber, but valves on the bottom and top of the inner cylinder work in such a manner so as to pump oil from the bottom of the outer reservoir cylinder up through the inner cylinder, past the piston, and out the top of the outer cylinder. Thus, the inner cylinder is always filled with oil and the erratic nature of the single cylinder shock abosrber is partially eliminated.
While possessing many advantages, two cylinder shock absorbers have several disadvantages. They have insufficient hydraulic fluid capacity so that at high rates of shock absorption they lose, through the effect of temperature on the viscosity of the available hydraulic fluid, their ability to absorb shocks. Shock absorption capability will then be dependent upon the rate at which the shock absorbers are worked. Further, the necessary air volume contained in the conventional two cylinder shock absorbers, to allow for the stroking of the shock absorbers shaft, will in time, allow the oil to become saturated with the air, at continued high rates of shock absorption, the oil on flow through the dampening orifices will not be as resistive as it would with no air in solution. A further disadvantage of two cylinder shock absorbers are that they operate with the air pocket in the upper-most section of the outer cylinder. This dictates that the shock absorber must be mounted so that the heaviest part, the two cylinders and the hydraulic fluid, is connected directly to the unsprung weight of the vehicle, thus adding to the need for shock absorption. A further disadvantage is that metallic particles of wear from within the shock absorber and solid particles taken within the shock absorber through the shaft seal on stroking of the shock absorber are retained within and continuously circulated throughout the shock absorber during use. These particles can plug the tiny orifice passages thus changing the characteristics of these shock absorbers.
Air-oil shock absorbers have been provided where, with a single cylinder, an air-oil floating piston isolates the oil of the shock absorber from a high pressure gas. The shock piston operates within the oil and the displacement of the oil caused by the entry and exit of the shock absorber rod are absorbed by compression and expansion of the gas as transmitted by the air-oil piston. However, air-oil shock absorbers also suffer from several disadvantages. Since the oil on the face side of the piston, between it and the free air-oil piston, is only contained by the air-oil piston, high rates of compressive force applied to the shock absorber will cause an air vapor cavity to form on the low pressure side of the piston because oil is unable to flow fast enough through the piston orifices. On a reversal of forces, from compressive to extensive, this vapor cavity will collapse, thereby causing a hydraulic shock of pressure to be transmitted throughout the hydraulic system. Eventually, the hydraulic seal on the piston rod will fail or the mechanical build-up of stresses on the tube, piston or cylinder will become too great for the system to withstand.